Add page to change a service's contact link

Added a page which lets users with the 'manage_service' permission change the
contact link for their service. There are no links to this page yet
since only services using document download will need to set a contact
link.

app/main/forms.py

@@ -28,7 +28,7 @@ from wtforms import (
     widgets,
 )
 from wtforms.fields.html5 import EmailField, SearchField, TelField
-from wtforms.validators import DataRequired, Length, Optional, Regexp
+from wtforms.validators import URL, DataRequired, Length, Optional, Regexp
 
 from app.main.validators import (
     Blacklist,
@@ -572,6 +572,14 @@ class ProviderForm(StripWhitespaceForm):
     priority = IntegerField('Priority', [validators.NumberRange(min=1, max=100, message="Must be between 1 and 100")])
 
 
+class ServiceContactLinkForm(StripWhitespaceForm):
+    url = StringField(
+        "URL",
+        validators=[DataRequired(message='Can’t be empty'),
+                    URL(message='Must be a valid URL')]
+    )
+
+
 class ServiceReplyToEmailForm(StripWhitespaceForm):
     email_address = email_address(label='Email reply to address', gov_user=False)
     is_default = BooleanField("Make this email address the default")